Natural gas is sometimes buried below the Earth's surface in shale reservoirs. To recover this natural gas, the shale reservoirs are hydraulically fractured with large volumes of frac fluid or gel to create a path for the natural gas to flow to the surface. Often, shale reservoirs are associated with ground water. When the natural gas flows to the surface for recovery, the natural water and the water in the frac fluid (sometimes referred to as frac flow back water) flows to the surface as well. The recovered natural water and the water used to frac the shale reservoir, along with oil in some cases, are separated from the natural gas. This water separated from the natural gas is referred to as produced water.
Produced water is often contaminated with concentrations of constituents requiring the produced water to be treated before it can be reused or discharged to the environment. Produced water typically includes natural contaminants that come from the subsurface environment. These contaminants include hydrocarbons from oil and inorganic salts. Produced water may also include manmade contaminants such as drilling mud, frac flow back water that includes spent fracturing fluids, including polymers and inorganic cross-leaking agents, polymer breaking agents, friction reduction chemicals, and artificial lubricants. These manmade contaminants are injected into wells as part of the drilling and production processes and recovered as contaminants in the produced water. In addition, the produced water may include contaminants that result from drilling mud. These contaminants may include acids and caustics such as soda ash, calcium carbonate, sodium hydroxide, and magnesium hydroxide. Furthermore, the contaminants in the produced water may include defoamers, emulsifiers, filtrate reducers, shale control inhibitors, deicers including methanol and other contaminants.
One of the challenges in treating produced water resulting from shale gas production and other natural gas production is to develop a practical and cost effective process that purifies the produced water such that it is suitable for use as frac water, in drilling mud, or which could be discharged to the environment.